The present invention relates to an RF amplifier bias circuit.
Initially, the biasing of transistor amplifiers was accomplished using purely resistive elements. For many reasons this is not the best approach to biasing broad band RF amplifiers. Typically the high frequency RF transistor, around which the RF amplifier stage is constructed, has its bias conditions [collector current (I.sub.c) and collector voltage (V.sub.c)] maintained by another, lower performance transistor circuit. This technique is known as Active Biasing, and is employed in many types of RF amplifiers.
FIG. 1 of the accompanying drawings shows a conventional, prior art active bias arrangement for an RF amplifier. An RF amplifier is indicated at 10 and an active bias circuit is indicated at 12.
The RF amplifier 10 includes an npn transistor 14 having a feedback stabilizing arrangement with a resistor R.sub.F, inductor L.sub.F, and capacitor C.sub.F. The transistor 14 has an inductor L.sub.C (RF choke) connected to its collector lead and has a grounded resistor R.sub.e connected to its emitter lead. The input and output RF signals are indicated at RF.sub.IN and RF.sub.OUT respectively.
The active bias circuit 12 includes a pnp transistor 16 with a resistor R.sub.b connected to its collector lead and a bias resistor R.sub.c connected to its emitter lead. The bias resistor R.sub.c is also connected to the inductor L.sub.c.
During operation, a fixed reference voltage V.sub.ref is supplied to the base of the transistor 16 and the supply voltage is V.sub.cc. The collector current I.sub.c of the transistor 14 is given by: ##EQU1##
The collector voltage V.sub.c of the transistor 14 is given by: EQU V.sub.c =V.sub.ref +0.7 V
Signal generators, and some other applications of broad band RF power amplifiers require operation in two modes:
i) Moderate output power (around 12 dBm) with good harmonic performance (-40 dBc). PA1 ii) High output power (around 20 dBm) with unspecified harmonic performance.
In signal generation, the RF amplifiers may be used to form part of an Automatic Level Control (ALC) loop in the prior art level feedback control system shown in FIG. 2. RF amplifiers 22,24 and 26, differential amplifier 28 and a modulator 30 generally comprise the feedback control system. The modulator 30 is typically a three PIN diode modulator. The RF amplifier 26 is connected to the negative input of the differential amplifier 28 via a detector diode 32. An RF level control voltage is supplied to the positive input of the differential amplifier 28. The RF power output from RF amplifier 26 is detected and compared with the RF level control voltage which is derived elsewhere in the instrument. The difference between the desired and detected RF level is used to generate an error signal which controls the modulator 30, changing the RF level input to the RF amplifier 22. In this way, the RF power out of the amplifier can be continuously varied through the two modes of operation mentioned above, as generally known to those skilled in the art.
Typical RF transistors used in an RF amplifier such as the Motorola MRF 581, have power output and harmonic performance which vary with V.sub.c and I.sub.c. In addition the product V.sub.c times I.sub.c constitutes the power dissipation in the transistor which is limited by the device packaging, heatsinking and specification. In conventional active biasing, a single value of V.sub.c and I.sub.c are chosen to give good performance in both operating modes while maintaining a low power dissipation. The relationship between V.sub.c, I.sub.c and transistor performance are such that increasing V.sub.c yields better harmonic performance with little effect on maximum output power, while increasing I.sub.c improves maximum output power before clipping and compression, with some degradation of harmonics.
FIGS. 3 and 4 illustrate the relationship between V.sub.c and harmonic performance and between I.sub.c and power output respectively in a typical prior art amplifier, such as for example, the Motorola MRF 581.